The microstructure effect on the fracture toughness of ferritic Ni-alloyed steels

Production of oil and gas in the Brazilian pre-salt faces several technical challenges and one of them that is a major concern is the presence of CO2 in high concentration. The aim of this work is to evaluate the fracture toughness of two nickel-containing steels as an alternative material to manufacture low-temperature toughness improved CO2 transporting pipelines for Enhanced oil recovery (EOR). Optical and scanning electron microscopies were employed to characterize the steels microstructures. Electron back-scattered diffraction was used to estimate the effective grain size and the density of high-angle grain boundaries. Fracture toughness was determined by the use of the crack tip opening displacement methodology. The results indicated that for the as-rolled condition the large islands of the microconstituent M/A in the 51/2 Ni steel had a detrimental effect on fracture toughness at −100 °C, while finer M/A particles and lower effective grain size with higher density of high-angle grain boundaries in the 9 Ni steel turned its fracture toughness practically temperature independent. Additionally, heat treatment (quenching and tempering) has the potential to dissolve the M/A hard particles and consequently improve fracture toughness at low temperature.